In recent years, use of trimethylsilane ((CH3)3SiH) has been extending as an interlayer insulating film and a film-forming raw material in the field of semiconductors.
As a method of producing trimethylsilane, it is general to use a suitable hydrogenation agent for reduction of trimethylchlorosilane ((CH3)3SiCl).
For example, a synthesis method that trimethylchlorosilane and lithium aluminum hydride (LiAlH4) are reacted in a solvent of dimethoxyethane (DME) has been disclosed (Non-patent Publication 1). Furthermore, a method that lithium hydride (LiH) (Patent Publication 1) or diethylaluminum hydride ((C2H5)2AlH) (Patent Publication 2) is used as a hydrogenation agent and a method that lithium aluminum hydride is used as a hydrogenation agent and an aromatic hydrocarbon series organic solvent is used as a solvent have been disclosed (Patent Publication 3).
Trimethylchlorosilane as a common raw material in these synthesis methods is contaminated usually with dozens to thousands ppm of methyltrichlorosilane (CH3SiCl3), dimethyldichlorosilane ((CH3)2SiCl2), silicon tetrachloride (SiCl4), etc. as impurities. When these impurities react with the hydrogenation agent, the corresponding silanes, that is, methylsilane (CH3SiH3), dimethylsilane ((CH3)2SiH2) or silane (SiH4) is produced. Furthermore, even in the case of not containing these chlorosilane series impurities at all, there occurs a disproportionate reaction in the process of the reaction with the hydrogenation agent. As a result, trimethylsilane is contaminated with these silanes as impurities. Alternatively, remains of the unreacted chlorosilane series impurities and impurities produced as by-products are also found.
Recently, there has been a demand for very high purity film forming raw materials in the semiconductor production. Using a synthesized trimethylsilane for semiconductors, the amount of these impurities must be reduced.
As a method of removing these impurities, distillation operation is usually used for refining. Besides, recrystallization, reprecipitation and sublimation can be used. Furthermore, there has been disclosed a method of utilizing activated carbon (Patent Publication 4) and a method of washing the gas with an absorbing solution (Patent Publication 5).
In the case of using distillation means, which is the most common, it was necessary to provide a distillation column having a high number of stages to completely remove dimethylsilane having a boiling point comparatively close to that of trimethylsilane, thereby causing a problem of not being economical. Furthermore, when trying to completely remove small amounts of the impurities having close boiling points by distillation, the loss of the product increases. This also leads to the yield reduction.
In contrast with this, it became possible to effectively remove the impurities by the above-mentioned method described in Patent Publication 4 utilizing activated carbon.